(Anglais) Magnesium alloy is the world’s lightest structural metal, with an extraordinary growth potential in view of the accelerating demand for light-weightening in portable devices and transportation (aerospace, automotive). However, this material has poor corrosion resistance properties that must be strongly improved. In order to protect magnesium pieces from aggressive environments, different kinds of protective layers can be elaborated. Anodizing, consisting in electrolytic growth of oxide layer at the magnesium surface, has mainly been developed for this purpose. By using alkaline electrolytes, metal dissolution and growth of the barrier layer occur on magnesium alloy substrate. The competition between the growth of the oxide, dissolution and oxygen evolution leads to a porous structure, with a thickness of few microns. However, such a layer does not provide a sufficient corrosion protection. To improve the corrosion resistance of the anodized layer, organic coating can be applied on the top of it, with enhanced adherence thanks to anchoring of the coating provided by the porous morphology.
This work aims at improving the understanding of the behavior of anodized coating for long immersion period in an aqueous electrolyte in absence of drying period. This anodic film has been prepared using hydroxide based electrolytes. The study focuses more specifically on the influence of the initial electrolyte pH on the dissolution of the anodic layer. Several electrolytes at different pH containing or not chlorides were used: an alkaline medium with or without chlorides, an electrolyte containing chlorides at natural pH. The morphology of anodic film has been investigated using Scanning electron microscopy (SEM). The electrochemical activity of the anodic film in these electrolytes was studied versus immersion using electrochemical impedance spectroscopy (EIS). Corrosion products formed in the surface were analysed using X-ray photoelectron spectroscopy (XPS).